This invention relates to a method for controlling fluid flow. In one aspect, the invention relates to a method for pumping and metering a corrosive, volatile, high density liquid at low flow rates.
Fluids can be moved through a conduit or tube by various methods including by centrifugal force, by volumetric displacement, by transfer of momentum from one fluid to another fluid, or by gravity.
Design criteria for selecting pumps for a particular service include design factors such as the physical and chemical properties of the liquid to be handled, the total head or pressure increase to be achieved, and other factors such as the service temperature range. Centrifugal pumps and positive displacement pumps, e.g., of the reciprocating or piston type, are widely used but are not particularly suited for pumping corrosive fluids in a production environment. These types of mechanical pumps require close tolerances and dynamic seals which break down over time in pumping corrosive fluids.
An acceleration of one fluid to transfer momentum to a second fluid is a principle commonly used in handling corrosive materials. Jet pumps such as jets and eductors are in this category. These types of momentum pumps, also sometimes called siphons or exhausters, usually are designed for pumping against a low head, e.g., pressures less than the pressure of the fluid used for pumping. The injector is a specialized type of jet pump which is operated by steam for boiler feed and other similar applications. In injectors, the pumped fluid is transferred to a space under the same pressure as the steam used in the injector.
Venturi nozzles are typically used to develop a suction from the momentum of a pumping fluid. The suction can be used to pull or pump a second fluid into the stream of flow.
Carburetors are well known liquid and gas contacting apparatus using a venturi to pump and to mix fuel with air. The resulting fuel mixture typically is used in an automobile internal combustion engine. A sub-atmospheric pressure on the engine side of the throttle provides the low pressure against which the carburetor operates as one type of eductor.
A good source of background information in this area is found in Perry's Chemical Engineering Handbook. For example, standard venturi meters are described, and standard dimensional proportions are recommended, including entrance and exit cone angles as well as throat length. Discharge coefficients for venturi meters are described in Perry's as the ratio of actual flow to theoretical flow taking into allowance stream contraction and frictional effects in the venturi. The discharge coefficient depends upon Reynolds Number and to a minor extent upon the size of the venturi, increasing with diameter.
Conventional fluid flow measurement is accomplished by velocity meters which can take the form of pitot tubes, radial vane meters, turbine flow meters, and others. Another fluid flow measurement device is a head meter which includes the venturi meter. The rate of discharge from the head meter can be calculated after determining pressure reduction, flow area at the constriction, fluid density, and the coefficient of discharge. Area meters are another fluid flow measurement device, and these include rotameters. Perry's describes rotameters as capable of covering over a tenfold range of flow, and by providing "floats" of different densities, a 200-fold range is practicable. Rotameters are available with pneumatic, electric, and electronic transmitters for actuating remote recorders, integrators, and automatic flow controllers.
Corrosive and volatile high density liquids are very difficult and expensive to pump and meter through conventional pumps and meters. The corrosive nature of the fluids is damaging to conventional mechanical pumps. The volatile nature of the fluid typically creates problems when service temperatures at the point of application are higher than the boiling point of the fluid. Such fluids are difficult to handle and meter as liquids through conventional flow meters particularly where the liquid has a high density and must be pumped at low flow rates.
It is an object of this invention to provide a method for the simple and accurate flow control of corrosive and volatile liquids.
A further object of this invention is to provide a method for pumping and metering a corrosive and volatile high density liquid at low flow rates.
It is another object of the present invention to provide a method for pumping a fluid by employing the momentum of a first fluid through an aspirator to pump a second fluid and to produce a controllable mixture of the first and second fluids in a predetermined composition independent of flow rates.
Other objects of this invention will become apparent from an inspection of the Summary and Detailed Description of the invention which follow.